1. Field of the Invention
The present invention relates to a controllable circuit protector and, more particularly, to a controllable circuit protector for power supplies with different voltages.
2. Description of the Related Art
With reference to FIG. 9, a regular fuse device 40 includes an input terminal I/P4, an output terminal O/P4, a control terminal C4, a first thermal fuse 41, a second thermal fuse 42 and a heating resistor R4. The first thermal fuse 41 is connected in series to the second thermal fuse 42. A series-connected node between the first thermal fuse 41 and the second thermal fuse 42 is electrically connected to one end of the heating resistor R4. A free end of the first thermal fuse 41 forms the input terminal I/P4, a free end of the second thermal fuse 42 forms the output terminal O/P4, and a free end of the heating resistor R4 forms the control terminal C4.
When the fuse device 40 is in use, the input terminal I/P4 is electrically connected to a power supply 50, the output terminal O/P4 is electrically connected to a load 60, and the control terminal C4 is electrically connected to a ground terminal GND through a switch SW4.
The switch SW4 is controlled to connect or disconnect the control terminal C4 according to a condition of the load 60. When the load 60 is in an abnormal state, the switch SW4 is controlled to connect to the control terminal C4 and the free end of the heating resistor R4 is connected to the ground terminal GND. Meanwhile, power supplied by the power supply 50 passes through the heating resistor R4 to heat up the heating resistor R4. The temperature rise of the heating resistor R4 causes the first thermal fuse 41 or the second thermal fuse 42 to melt.
The temperature rising speed of the heating resistor R4 is determined by the heating power of the heating resistor R4. After the temperature of the heating resistor R4 exceeds the meltdown temperature of the first thermal fuse 41 or the second thermal fuse 42, meltdown of the first thermal fuse 41 or the second thermal fuse 42 then happens. An equation for calculating the heating power of the heating resistor R4 is expressed as follows:
  P  =            V      2        R  where P is the heating power of the heating resistor R4; V is the voltage value provided by the power supply 50; and R is the resistance value of the heating resistor R4.
When the load is detected to be at an abnormal state, the first thermal fuse 41 or the second thermal fuse 42 must melt down within a specific period of time before it is too late to protect the load 60. Hence, when the resistance value R of the heating resistor R4 is a fixed value, the time for melting down the first thermal fuse 41 or the second thermal fuse 42 is positively correlated with the heating power P of the heating resistor R4. When the heating power P of the heating resistor R4 increases, the time for melting down the first thermal fuse 41 or the second thermal fuse 42 is shortened or is negatively correlated with the heating power P of the heating resistor R4. Also, the heating power P of the heating resistor R4 is positively correlated with the square of the voltage value V provided by the power supply 50. Therefore, the meltdown time of the first thermal fuse 41 or the second thermal fuse 42 is negatively correlated with the square of the voltage value V provided by the power supply 50.
In other words, when the voltage value V provided by the power supply increases, the meltdown time of the first thermal fuse 41 or the second thermal fuse 42 is shortened.
However, when manufacturers of the regular fuse device 40 produce the fuse device 40, same type of heating resistors R4 is used exclusively. In other words, the resistance value R of the heating resistor R4 is a fixed value. Thus, the fuse device 40 is only applicable to the power supplies at a specific voltage value and up. When the voltage value V of the power supply 50 is lower than the specific voltage value and the load 60 is at an abnormal state, the switch SW4 connects the control terminal C4, the first thermal fuse 41 or the second thermal fuse 42 has a long meltdown time arising from the lower heating power P of the heating resistor R4 and thus fails to melt down in time to protect the load 60, and the cost is damage to the load 60.
For example, when the fuse device 40 manufactured is applied to the power supply 50 at 220V, the load 60 is at an abnormal state and the switch SW4 connects to the control terminal C4, the voltage value 220V of the power supply 50 heats up the heating resistor R4 for the first thermal fuse 41 or the second thermal fuse 42 to have timely meltdown. However, when the fuse device 40 is applied to the power supply 50 at 110V and the switch SW4 connects to the control terminal C4, the voltage value 110 of the power supply 50 results in a reduced heating power P at the heating resistor R4, such that the first thermal fuse 41 or the second thermal fuse 42 fails to melt down in time because of prolonged meltdown time of the first thermal fuse 41 or the second thermal fuse 42 and the prolonged meltdown time also causes damage to the load 60.
As a result, the fuse device 40 made for 220V power supply is unable to be used for 110V power supply. Suppose that the fuse device 40 needs to be used with 110V power supply. Manufacturers must redesign the heating resistor R4 in the fuse device 40 with a different resistance value, meaning that the fuse device must be rebuilt. To manufacturers of the fuse device, different fuse devices dedicated to power supplies with different voltage values relatively increase the cost in production. As current power equipment is operated in a working range of 90˜260V, the regular fuse device 40 fails to meet the requirement, does not work with AC (Alternating Current) power supplies, and can only work with DC (Direct Current) power supplies under 30V.